Cabs of tractors, trucks and the like of course need ventilation to keep the environment satisfactory for what can be very long hours of use during various seasons of the year. For this purpose it is necessary to draw air into the cab for passage through a heating or air conditioning system for properly ventilating the cab with air of suitable temperature. Many cabs have a single air inlet located in a suitable place for example adjacent the roof line and projecting outwardly, forwardly or rearwardly of the cab with an opening facing downwardly. In many cases a dust filter is positioned at the opening so that the air drawn into the cab is free from dust since much farmwork or truck driving is done in dusty areas where the wheels of the vehicle tend to generate dust which is drawn into the cab.
U.S. Pat. Nos. 3,180,244 (Mescher), 4,120,527 (Lawrence), 3,868,896 (Doll) and 4,492,151 (Mattei) show examples of cab design incorporating various filter arrangements.
In particular Mattei discloses an arrangement including a first dust filter following which there is provided an expansion space from which the air passes through a chemical filter of the type using activated charcoal. This arrangement is very bulky and requires a significant number of parts and thus leads to an increased cost.
In addition there are already on the market large numbers of tractor cabs with ventilation systems. In one example there is provided a downwardly directed inlet opening adjacent the roof line of the cab from an inlet chamber which projects rearwardly from the cab. The inlet opening which may be located and oriented in other arrangements is generally rectangular and is closed by a hinged dust filter which defines a rectangular casing shaped to close the opening. The dust filter is arranged to be latched up against a flange surrounding the opening and defining an abutment surface facing downwardly from the opening with a resilient sealing means between the abutment surface and the dust filter so that any air drawn into the chamber passes through the dust filter. Such an arrangement does not include a chemical filter and to date no proposal has been made for simply incorporating a chemical filter into that system in view of the difficulty of placing the filter in the chamber downstream of the filter and upstream of the fan. However more concern has been generated in recent years concerning the toxic chemicals with which farmers work and particularly the various sprays which are currently used. While such sprays are harmless in the finished food product, in concentration there can be significant danger and accordingly there is an increased need and desire to provide a filter which will extract such toxic chemicals from the air as it is drawn into the cab.